There is a trend in the United States in recent years in mandating that a tire air pressure monitoring system be mounted in automotive vehicles. Conventionally, there are systems that do not require a battery because of the use of a surface acoustic wave (SAW) device as a method for directly measuring the air pressure of a tire.
An RFID (Radio Frequency Identification) technique is known in which a wireless connection is provided between a tire-mounted pressure sensor and a controller on the vehicle body. In a configuration that uses this RFID technique, an RFID transponder (RF tag) containing an inflation pressure sensor is disposed in the internal space of the tire, and a reader/writer is disposed on the vehicle body. For the sake of convenience, methods for achieving the wireless connection between the two members are categorized, in accordance with fluctuation frequency and transmission distance of the electromagnetic field, into electromagnetic wave methods, electromagnetic induction methods, and electromagnetic coupling methods. However, the methods are performed using fluctuations in the electromagnetic field and are essentially physically the same.